An often used piece of off-highway construction equipment is a bottom dump truck consisting of a tractor or prime mover and a towed, load-carrying semi-trailer. The semi-trailer has a pair of abutting dump doors in its bottom to permit earth or similar loads carried in the trailer to be dumped between the wheels thereof as the truck moves along. The use of the typical bottom dump truck is limited to relatively smooth terrain and fairly stable surfaces, since only the tractor portion of the truck provides a force to move the truck, and the towed trailer, when fully loaded, can easily become bogged down in sandy or damp soils, for example.
In other types of wheeled, off-highway construction equipment, it has been proposed to extend the range of conditions under which such equipment can operate by providing all-wheel drive through two drive engines, one mounted on the front to drive the front wheels and one mounted on the rear to drive the rear wheels. The two-engine, all-wheel drive arrangement is often seen in construction equipment termed "scrapers" or "pans" that can be run along a surface to scrape earth into a load-carrying hopper. When full, the hopper is closed by a large blade-like closure and the body of the scraper is raised from the ground to permit the scraper to move away from the construction site. The use of two engines greatly increases the production and operating costs of a piece of off-highway equipment and is probably justified for scrapers only because a single engine scraper is generally pushed by a bulldozer, for example, in order to build up a full hopper load. A two-engine, all-wheel drive scraper, on the other hand, has sufficient power and traction to scrape up a full load of earth generally without the assistance of a pushing bulldozer, thus freeing the bulldozer and its operator for other work.
Outside the construction field, general purpose off-highway vehicles have been proposed in which a single engine drives all of the wheels of both front and rear sections of the vehicle, the two vehicle sections being articulated together. Off-highway, general purpose vehicles are described and illustrated in the Holland U.S. Pat. No. 3,630,302 and in the Mueller et al. U.S. Pat. No. 3,802,526. It has also been proposed to have a drive engine mounted on the trailer section of a tractor-trailer highway vehicle to drive both front and rear wheels of the vehicle. Examples of such tractor-trailer vehicles are described and illustrated in the Brown U.S. Pat. No. 2,648,392 and the Lewis U.S. Pat. No. 3,454,123.
In order to meet the requirements of the strip mining industry, for example, it would be desirable to have a bottom dump truck having all-wheel drive so that the truck could travel over any type of terrain. While it might initially seem possible to construct a bottom truck similar to the vehicles illustrated in the Holland and Mueller et al. patents, the bottom dump feature raises problems not encountered in the Holland and Mueller et al. vehicles, which utilize load-carrying beds or boxes having solid floors. Specifically, the drive shaft, which extends between the two vehicle sections and which runs under the rear vehicle section in both the Holland and Mueller et al. vehicles, cannot be run under the rear section of a bottom dump truck without interfering with the dump doors. If one were to adopt the construction of the prior art scrapers, however, and utilize separate engines for the front and rear wheels, the result would be an excessively costly and possibly overpowered vehicle which, unlike a dual engine scraper, could not justify its high costs by offering the advantage of freeing an additional piece of equipment, such as a bulldozer. Similarly, to couple a prime mover in one vehicle section with hydraulic or electric motor(s) in the other section by means of hydraulic or electric conduits, which could be routed laterally of the dump doors to avoid interference therewith, would involve a complex and costly drive train, as compared with conventional drive shafts and differential and planetary gearing.